JP2011249008A - Separator for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator for a lithium ion secondary battery which has no pinhole, improves interlayer detachability between a layer obtained by electrospinning and a base material layer, improves heat resistance and also improves internal resistance.SOLUTION: A separator for a lithium ion secondary battery has a base material layer including a synthetic resin short fiber and fibrillated lyocell fiber as essential components, and an ultra fine fiber layer obtained by electrospinning is stacked at least on one surface.

Description

  The present invention relates to a separator for a lithium ion secondary battery that can be suitably used for a lithium ion secondary battery such as a lithium ion secondary battery or a lithium ion polymer secondary battery.

  With the recent spread of portable electronic devices and higher performance, secondary batteries having high energy density are desired. As this type of battery, a lithium ion secondary battery using an organic electrolyte (non-aqueous electrolyte) has attracted attention. This lithium ion secondary battery has an energy density of about 3.7 V, which is about three times that of an alkaline secondary battery, which is a conventional secondary battery, and thus has a high energy density. Since a water-based electrolyte cannot be used, a non-aqueous electrolyte having sufficient oxidation-reduction resistance is used. Since non-aqueous electrolytes are flammable, there is a risk of ignition and the like, and careful attention is paid to safety in their use. There are several possible cases of exposure to fire and other hazards, but overcharging is particularly dangerous.

  In order to prevent overcharging, current non-aqueous secondary batteries are charged at a constant voltage and a constant current, and the battery is equipped with a precise IC (protection circuit). The cost required for this protection circuit is large, and it is a factor that increases the cost of non-aqueous secondary batteries.

  When overcharging is prevented by the protection circuit, it is naturally assumed that the protection circuit does not operate well, and it is difficult to say that it is intrinsically safe. The current non-aqueous secondary battery has a safety circuit / PTC element equipped and a separator with a thermal fuse function for the purpose of destroying the battery safely when overcharged. Has been made. However, even if equipped with the above-mentioned means, depending on the overcharge conditions, the safety during overcharge is not guaranteed, and in fact, non-aqueous secondary battery ignition accidents Is still happening.

  As the separator, a film-like porous film made of a polyolefin such as polyethylene is often used. When the temperature inside the battery reaches around 130 ° C., it melts and closes the micropores, thereby transferring lithium ions. Although there is a thermal fuse function (shutdown function) that cuts off the current and shuts off the current, it is suggested that if the temperature further increases due to some situation, the polyolefin itself melts and short-circuits, causing a thermal runaway. In view of this, a heat-resistant separator that does not melt and shrink even at temperatures close to 200 ° C. has been developed.

  Then, the example which gives heat resistance by compounding, such as formation of the porous film by content of the filler particle in a nonwoven fabric or a woven fabric, and resin surface coating, is reported (for example, refer patent document 1). However, the non-woven fabric used as the base material has large pores and low surface smoothness, so the surface variation when combined by surface coating is large, and composites such as filler particles and resins As a result, there are problems such as limited fields that can be used and poor battery characteristics such as internal resistance.

  In addition, there has been reported an example in which layers made of fine fibers of a specific size spun by an electrostatic spinning method are laminated on both surfaces of a network structure sheet such as a nonwoven fabric (for example, see Patent Document 2). However, in such a separator, it is necessary to increase the basis weight of the fine fiber layer by the electrospinning method in order to sufficiently seal the excessive gap between the fibers of the network structure sheet. There was a problem of getting worse. Furthermore, since the fine fiber layer spun by the electrostatic spinning method and the mesh sheet cannot be sufficiently bonded, there is a problem that the interlaminar strength is insufficient and cannot be used in actual use.

  In addition, an example is described in which, after superimposing a low-weight fine fiber layer and a wet nonwoven fabric layer by an electrostatic spinning method, both layers are bonded and integrated by heating and pressurization (see, for example, Patent Document 3). . In this method, from the viewpoint of strength, there is a problem that not only the fine fiber layer with a low basis weight can be stably obtained by the electrostatic spinning method, but also the fine fiber layer and the wet nonwoven fabric layer are easily delaminated. .

JP 2005-536857 A JP 2006-92829 A International Publication No. 2006/049151 Pamphlet

  An object of the present invention is to provide a lithium ion secondary battery separator having no pinholes, excellent delamination between the layer obtained by the electrospinning method and the base material layer, excellent heat resistance and excellent internal resistance. It is to provide.

  As a result of diligent research to solve the above-mentioned problems, the present inventors have a base material layer containing essential synthetic resin short fibers and fibrillated lyocell fibers as an essential component, and obtained by an ultraspinning method. The inventors have found a lithium ion secondary battery separator characterized in that an ultrafine fiber layer is directly deposited and laminated on the base material layer.

  The separator for a lithium ion secondary battery of the present invention has a base layer excellent in heat resistance containing a synthetic resin short fiber and a fibrillated lyocell fiber as essential components, and on at least one side of the base layer, It is characterized in that super extra fine fiber layers obtained by an electrospinning method are laminated. In the base material layer, the fibrillated lyocell fibers are intertwined with the synthetic resin short fibers and have appropriate voids, so that the ultrafine fibers obtained by the electrospinning method are fibrillated lyocell fibers in the voids of the base material layer. And the delamination between the base material layer and the ultrafine fiber layer is less likely to occur. Furthermore, in the base material layer, the fibrillated lyocell fiber is entangled with the synthetic resin short fiber to form a dense structure, thereby causing an internal short circuit even if the ultrafine fiber layer obtained by the electrospinning method is a thin film. As a result, it has a feature of excellent internal resistance. In other words, the lithium ion secondary battery separator of the present invention has excellent heat resistance, strong interlaminar strength between the ultrafine fiber layer obtained by the electrostatic spinning method and the base material layer, and excellent internal resistance. .

  Hereinafter, the separator for a lithium ion secondary battery of the present invention will be described in detail. The separator for a lithium ion secondary battery of the present invention is obtained by an electrospinning method on at least one surface of a base material layer containing synthetic resin short fibers and fibrillated lyocell fibers as essential components. It consists of a laminated nonwoven fabric having a super extra fine fiber layer. The resins that make up the synthetic resin short fibers used in the base material layer include polyester resins, polyvinyl acetate resins, ethylene-vinyl acetate copolymer resins, polyamide resins, acrylic resins, polyolefin resins, and polychlorinated resins. Vinyl resin, polyvinylidene chloride resin, polyvinyl ether resin, polyvinyl ketone resin, polyether resin, polyvinyl alcohol resin, diene resin, polyurethane resin, phenol resin, melamine resin, furan resin, Examples include urea resins, aniline resins, unsaturated polyester resins, alkyd resins, fluorine resins, and silicone resins. Among these, it is preferable to use the fiber which consists of polyester-type resin and acrylic resin excellent in heat resistance.

  Examples of the polyester resin include polyethylene terephthalate, polybutylene terephthalate, polytrimethylene terephthalate, polyethylene naphthalate, polybutylene naphthalate, and polyethylene isophthalate. Among these, when used for a separator for a lithium ion secondary battery, a polyethylene terephthalate system having excellent heat resistance is preferable.

  Acrylic resin is made of 100% acrylonitrile polymer, and acrylonitrile is copolymerized with (meth) acrylic acid derivatives such as acrylic acid, methacrylic acid, acrylic ester, methacrylic ester, vinyl acetate, etc. And the like.

  The synthetic resin short fiber may be a fiber (single fiber) made of a single resin, or may be a fiber (composite fiber) made of two or more kinds of resins. Moreover, the synthetic resin short fiber contained in the separator for lithium ion secondary batteries of the present invention may be one kind or a combination of two or more kinds.

  As the synthetic resin short fiber, a heat-fusible short fiber that functions as a binder may be used. Examples of the heat-fusible short fiber include a core-sheath type, an eccentric type, a side-by-side type, a sea-island type, an orange type, a multi-bimetal type composite fiber, or a single fiber. One-component type is preferable, and it is particularly preferable to use unstretched polyester short fibers. Moreover, it is preferable to use a core-sheath-type polyester short fiber from the point of obtaining uniform and high strength.

  The fineness of the synthetic resin short fibers is preferably 0.007 to 0.8 dtex, more preferably 0.02 to 0.6 dtex, and further preferably 0.04 to 0.3 dtex. When the fineness of the synthetic resin short fibers exceeds 0.8 dtex, the number of fibers in the thickness direction decreases, and thus the required denseness may not be ensured. When the fineness of the synthetic resin short fiber is less than 0.007 dtex, stable fiber production may be difficult.

  The fiber length of the synthetic resin short fiber is preferably 1 mm or more and 7 mm or less, more preferably 1 mm or more and 5 mm or less, and further preferably 1 mm or more and 3 mm or less. When the fiber length exceeds 7 mm, the formation may be poor. On the other hand, when the fiber length is less than 1 mm, the mechanical strength of the base material layer becomes low, and the base material layer may be damaged when the super extra fine fiber layer obtained by the electrostatic spinning method is laminated.

  “Lyocell” of fibrillated lyocell fiber is a term defined in ISO standards and Japanese JIS standards, and refers to “cellulose fibers obtained by spinning directly in an organic solvent without spinning through cellulose derivatives”. is there.

  Lyocell fibers are beater and dispersion equipment such as beaters, PFI mills, single disc refiners (SDR), double disc refiners (DDR), ball mills and dyno mills used to disperse and pulverize pigments, etc. Can be fibrillated. It is desirable to use lyocell fibers that are optimally fibrillated using these dispersing facilities.

  The fibrillated lyocell fiber used for the base material layer of the lithium ion secondary battery separator of the present invention preferably has a modified freeness of 0 to 250 ml, more preferably 0 to 160 ml. When the modified freeness exceeds 250 ml, the refinement process is insufficient, the fiber division does not proceed sufficiently, and the ratio of the fiber diameter remaining large increases, so that a large through hole is formed in the base material layer. In some cases, the ultra-fine fiber layer obtained by the electrospinning method may come through. Furthermore, the length weighted average fiber length of the fibrillated lyocell fiber used for the base material layer of the separator for a lithium ion secondary battery of the present invention is more preferably 0.20 to 2.00 mm, and 0.40 to 1.60 mm. Is more preferable. When the length-weighted average fiber length is less than 0.20 mm, the fibrillar lyocell fiber may fall off from the base material layer, and the ultrafine fiber layer obtained by the electrostatic spinning method may be broken due to fuzzing. On the other hand, if it is longer than 2.00 mm, the fibers of the base material layer tend to be entangled, and unevenness in the formation and unevenness in thickness may occur.

  The modified freeness in the present invention is an 80-mesh wire mesh (PULP AND PAPER RESEARCH INSTITUTE OF CANADA) having a wire diameter of 0.14 mm and an aperture of 0.18 mm as a sieve plate and a sample concentration of 0.1%. Except for the above, it means the modified freeness measured in accordance with JIS P8121, and is simply expressed as “modified freeness” unless otherwise specified. The length-weighted average fiber length of the solvent-spun cellulose fiber in the present invention can be obtained by using a deflection characteristic obtained by applying laser light to the fiber, and can be measured using a commercially available fiber length measuring device. .

  In the base material layer of the separator for a lithium ion secondary battery of the present invention, the content mass ratio between the synthetic resin short fibers and the fibrillated lyocell fibers is preferably 90/10 to 20/80, and 80/20 to 30/70. Is more preferable, and 70/30 to 40/60 is more preferable. When the content ratio of the fibrillated lyocell fiber is less than 10% by mass, sufficient heat resistance may not be obtained, and denseness and uniformity may not be improved. Moreover, when the content ratio of the fibrillated lyocell fiber exceeds 80% by mass, the base material may be damaged when the super fine fiber layer obtained by the electrostatic spinning method is laminated.

  The base material layer of the lithium ion secondary battery separator of the present invention has a synthetic resin short fiber and a modified freeness of 0 to 250 ml, and a length weighted average fiber length of 0.20 to 2.00 mm. Fibers other than the fibrillated lyocell fibers may be contained. For example, natural cellulose fibers, pulped and fibrillated natural cellulose fibers, short fibers of solvent-spun cellulose, fibrils made of synthetic resin, pulped products, fibrillated products, inorganic fibers, modified fibrillation with a freeness of more than 250 ml Examples include lyocell fibers and fibrillated lyocell fibers having a length-weighted average fiber length of less than 0.20 mm or more than 2.00 mm. The modified freeness of the natural cellulose fiber pulped product or fibrillated product is preferably 0 to 400 ml. Examples of the inorganic fiber include glass, alumina, silica, ceramics, and rock wool. When inorganic fiber is contained, it is preferable because the heat-resistant dimensional stability and puncture strength of the lithium ion secondary battery separator are improved.

Weight per unit area of the base material layer is preferably 3.0~30.0g / ^{m 2,} more preferably 5.0~20.0g / ^{m 2,} more preferably 6.0~16.0g / ^{m 2.} If it exceeds 30.0 g / m ² , the base material alone occupies most of the separator, and it becomes difficult to obtain the effect of lamination with the ultrafine fiber layer obtained by the electrospinning method. If it is less than m ² , it will be difficult to obtain sufficient strength, and a large variation may easily occur on the surface after lamination with the ultrafine fiber layer. The basis weight means the basis weight based on the method defined in JIS P 8124 (paper and paperboard—basis weight measurement method).

  The electrostatic spinning method used for the separator for the lithium ion secondary battery of the present invention is a target in which, when a high voltage is applied to a polymer stock solution in which a raw material polymer is dissolved or melted, the charged polymer stock solution is split and grounded. This is a method utilizing the fact that ultrafine fibers are deposited on an ultrafine fiber layer. When an organic solvent solution is used as a polymer stock solution, the organic solvent is easily evaporated and removed at the stage of fiber formation and refinement, and when a melt is used as a polymer stock solution, it is below the melting temperature. And is deposited on a base material layer disposed on a collection belt or a collection sheet installed at a predetermined interval from the polymer stock solution supply unit. As the polymer stock solution supply section for supplying the polymer stock solution to the spinning space, a method using a plurality of general spinning nozzles may be employed, or a method using no spinning nozzle may be employed.

  The polymer of the ultrafine fiber obtained by the electrospinning method is not particularly limited as long as it can be made into a solution or meltable, and can be used. Examples of such polymers include polyvinyl alcohol resins, acrylic resins, polyimide resins, polyamide resins, fluorine resins, polystyrene resins, polyester resins, cellulose, cellulose acetate, polyvinyl chloride resins, and polylactic acid. Various polymers that can be melted or dissolved in an appropriate solvent such as a resin can be used, and copolymers and mixtures thereof can also be used. The polymer stock solution may be mixed with an emulsion such as a synthetic resin or an organic or inorganic powder. Among these, when used for a separator for a lithium ion secondary battery, a polyvinyl alcohol resin, an acrylic resin, a fluorine resin, a polyimide resin, a polystyrene resin, a cellulose, and a polyamide resin that are excellent in heat resistance. And polyester resins are preferred, and polyvinyl alcohol resins, acrylic resins, and fluorine resins are more preferred.

  The solvent for dissolving the polymer is not particularly limited. For example, (a) highly volatile acetone, ethanol, methanol, isopropanol, chloroform, toluene, tetrahydrofuran, water, benzene, benzyl alcohol , 1, propanol, 4-dioxane, carbon tetrachloride, cyclohexane, cyclohexanone, methylene chloride, phenol, pyridine, trichloroethane, acetic acid, etc. (b) N, N-dimethylformamide, dimethyl sulfoxide, N , N-dimethylacetamide, 1-methyl-2-pyrrolidone, ethylene carbonate, propylene carbonate, dimethyl carbonate, acetonitrile, diethyl ether, diethyl carbonate, 1,2-dimethoxyethane, 1, - dimethyl-2-imidazolidinone, 1,3-dioxolane, ethyl methyl carbonate, methyl propionate, and the like 2-methyltetrahydrofuran. If a mixed solvent in which a highly volatile solvent and a solvent having relatively low volatility are mixed is used, the volatility of the solvent can be increased and the viscosity of the polymer stock solution can be reduced. By increasing the discharge amount from the member, the productivity of the fine fiber layer by the electrostatic spinning method can be improved.

Basis weight of the microfiber layer is preferably 0.1~15.0g / ^{m 2,} more preferably 0.3~10.0g / ^{m 2,} more preferably 0.3~8.0g / ^{m 2.} If it exceeds 15.0 g / m ² , the ratio of the ultrafine fiber layer is increased, and the internal resistance may be deteriorated. If it is less than 0.1 g / m ² , the gap in the base material layer cannot be filled. Short circuit may occur easily.

  The method for laminating the base material layer and the ultrafine fiber layer of the separator for the lithium ion secondary battery of the present invention is not particularly limited, but the ultrafine fiber layer obtained by the electrostatic spinning method is directly bonded to the substrate. A method of depositing and laminating on the material layer sheet is preferable. Once a sheet made of ultrafine fibers obtained by an electrospinning method is manufactured, the superfine fiber layer sheet and the base material layer sheet are bonded together with an adhesive, or the super fine fiber layer or the base material layer There is also a method of using fibers made of a heat-melting resin and laminating both layers by heating and pressurizing at a temperature equal to or higher than the melting point of the heat-melting resin, but in these laminating methods, the base material layer, the ultrafine fiber layer, The peel strength may be inferior.

Although the thickness of the separator for lithium ion secondary batteries of this invention is not specifically limited, 3-50 micrometers is preferable, 6-40 micrometers is more preferable, 8-30 micrometers is further more preferable. If it exceeds 50 μm, the film thickness becomes too thick and the internal resistance may be inferior, and if it is less than 3 μm, the strength of the separator becomes too low and may be damaged when assembled in a battery. In addition, thickness means the value measured by the method prescribed | regulated to JISB7502, ie, the value measured with the outside micrometer at the time of 5N load. Also, the basis weight of the separator for lithium ion secondary battery of the present invention is preferably 3.1~40.0g / ^{m 2,} more preferably 5~30.0g / ^{m 2,} more preferably 6~20g / ^{m 2} . If it exceeds 40.0 g / m ² , the film thickness becomes too thick, the denseness becomes too dense, and the internal resistance may deteriorate, and if it is less than 3.1 g / m ² , sufficient strength is obtained. It is difficult to obtain a short circuit, and a short circuit is likely to occur, which may not be suitable for actual use as a separator.

  In the separator for a lithium ion secondary battery of the present invention, as a method for producing a sheet serving as a base material layer, a method of forming a fiber web and bonding, fusing, and entanglement of fibers in the fiber web can be used. . The obtained nonwoven fabric may be used as it is or may be used as a laminate comprising a plurality of sheets. Examples of the method for producing the fiber web include a dry method such as a card method and an air array method, a wet method such as a papermaking method, a spunbond method, and a melt blow method. Among these, the web obtained by a wet method is uniform and dense, and can be suitably used as a separator for a lithium ion secondary battery. Furthermore, when manufacturing a base material layer having a multi-layer structure, an integrated base material layer without peeling between each of the plurality of layers can be manufactured by using a wet-making method. Wet-making method is a papermaking slurry in which fibers are dispersed in water to form a uniform papermaking slurry, and this papermaking slurry is made using a papermaking machine having at least two wires such as a circular net, a long net, and an inclined type. This is a method for obtaining a fibrous web.

  The lithium ion secondary battery separator of the present invention may be further combined, and the combination is not particularly limited, but is laminated with a porous film, impregnated with filler particles, or surface. Examples thereof include coating, impregnation with a polymer resin, and surface coating.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited to a present Example. All parts and percentages in the examples are based on mass unless otherwise specified.

[Production of fibrillated lyocell fiber]
Using a double disc refiner, fibrillated lyocell fibers (fiber diameter 15 μm, fiber length 4 mm, manufactured by Cauters) were repeatedly treated 50 times to obtain fibrillated lyocell fibers. The modified freeness of the fibrillated lyocell fiber was 100 ml, and the length weighted average fiber length was 0.78 mm. The modified freeness is a variable measured in accordance with JIS P8121, except that an 80-mesh wire mesh having a wire diameter of 0.14 mm and an aperture of 0.18 mm is used as the sieve plate, and the sample concentration is 0.1%. It means the freeness of the law. Further, the length-weighted average fiber length of the fibrillated lyocell fiber in the present invention is a numerical value obtained by utilizing a deflection characteristic obtained by applying a laser beam to the fiber, and is measured using a commercially available fiber length measuring instrument. be able to.

[Preparation of polymer stock solution A]
Polyvinyl alcohol (polymerization degree 1,900, saponification degree 94.0-96.0 mol%) was dissolved in pure water to prepare a polymer stock solution B for electrospinning having a concentration of 10% by mass.
[Preparation of polymer stock solution B]
Polyvinylidene fluoride (mass average molecular weight 300,000) was dissolved in N, N-dimethylformaldehyde to prepare a polymer stock solution A for electrospinning having a concentration of 10% by mass.
[Preparation of polymer stock solution C]
Homopolyacrylonitrile (mass average molecular weight 350,000) was dissolved in dimethylformaldehyde to prepare a polymer stock solution C for electrospinning having a concentration of 10% by mass.

Example 1
For the base material layer (1), 20 parts of oriented and crystallized polyethylene terephthalate (PET) short fibers with a fineness of 0.1 dtex and fiber length of 3 mm, unstretched PET heat fusion with a fineness of 0.2 dtex and fiber length of 3 mm 40 parts of woven short fiber and 40 parts of fibrillated lyocell fiber prepared by the above method are mixed together, disaggregated in pulper water, and a uniform papermaking slurry (1% concentration) is prepared under stirring by an agitator did. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 25 μm was produced and used as a base material layer (1). Next, the polymer stock solution A is sprayed on one side of the base material layer (1) using an electrostatic spinning method (applied voltage: 12 kV, distance of 15 cm between the spinneret and the base material layer (1)), and the thickness is about 4 μm. The ultrafine fiber layer of was formed. In this state, since the crystallinity is low and soluble in water, heat treatment (150 ° C., 3 min) is then performed, supercalender treatment is further performed, and a lithium ion secondary battery separator (weight is 13.2 g / m). ² and a thickness of 21 μm).

(Example 2)
For the base material layer (1), 20 parts of oriented and crystallized polyethylene terephthalate (PET) short fibers with a fineness of 0.1 dtex and fiber length of 3 mm, unstretched PET heat fusion with a fineness of 0.2 dtex and fiber length of 3 mm 40 parts of woven short fiber and 40 parts of fibrillated lyocell fiber prepared by the above method are mixed together, disaggregated in pulper water, and a uniform papermaking slurry (1% concentration) is prepared under stirring by an agitator did. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 25 μm was produced and used as a base material layer (1). Next, the polymer stock solution B is sprayed on one side of the base material layer (1) using an electrostatic spinning method (applied voltage: 15 kV, distance of 15 cm between the spinneret and the base material layer (1)), and the thickness is about 3 μm. After forming the ultrafine fiber layer, a super calender treatment was performed to obtain a lithium ion secondary battery separator (weight per unit area: 12.5 g / m ² , thickness: 20 μm).

(Example 3)
For the base material layer (1), 20 parts of oriented and crystallized polyethylene terephthalate (PET) short fibers with a fineness of 0.1 dtex and fiber length of 3 mm, unstretched PET heat fusion with a fineness of 0.2 dtex and fiber length of 3 mm 40 parts of woven short fiber and 40 parts of fibrillated lyocell fiber prepared by the above method are mixed together, disaggregated in pulper water, and a uniform papermaking slurry (1% concentration) is prepared under stirring by an agitator did. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 25 μm was produced and used as a base material layer (1). Next, the polymer stock solution C is sprayed on one side of the base material layer (1) using an electrostatic spinning method (applied voltage: 12 kV, distance of 10 cm between the spinneret and the base material layer (1)), and the thickness is about 3 μm. After forming the ultrafine fiber layer, a super calender treatment was performed to obtain a lithium ion secondary battery separator (weight per unit: 12.9 g / m ² , thickness: 20 μm).

Example 4
For the base material layer (2), 20 parts of acrylic short fibers having a fineness of 0.1 dtex and a fiber length of 3 mm, 40 parts of unstretched PET heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm, and the above method 40 parts of the fibrillated lyocell fiber produced in the above were mixed together, disaggregated in pulper water, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 26 μm was produced and used as a base material layer (2). Next, the polymer stock solution A is sprayed on one side of the base material layer (2) using an electrostatic spinning method (applied voltage: 12 kV, distance of 15 cm between the spinneret and the base material layer (2)), and the thickness is about 4 μm. The ultrafine fiber layer of was formed. In this state, since the crystallinity is low and soluble in water, a heat treatment (150 ° C., 3 min) is performed, followed by a super calendering process to obtain a separator for a lithium ion secondary battery (weight per unit: 13.5 g / m ² , thickness 20 μm).

(Example 5)
For the base material layer (2), 20 parts of acrylic short fibers having a fineness of 0.1 dtex and a fiber length of 3 mm, 40 parts of unstretched PET heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm, and the above method 40 parts of the fibrillated lyocell fiber produced in the above were mixed together, disaggregated in pulper water, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 26 μm was produced and used as a base material layer (2). Next, the polymer stock solution B is sprayed on one surface of the base material layer (2) using an electrostatic spinning method (applied voltage: 15 kV, distance of 15 cm between the spinneret and the base material layer (2)), and the thickness is about 3 μm. After forming the ultrafine fiber layer, a super calender treatment was performed to obtain a lithium ion secondary battery separator (weight per unit: 12.4 g / m ² , thickness: 19 μm).

(Example 6)
For the base material layer (2), 20 parts of acrylic short fibers having a fineness of 0.1 dtex and a fiber length of 3 mm, 40 parts of unstretched PET heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm, and the above method 40 parts of the fibrillated lyocell fiber produced in the above were mixed together, disaggregated in pulper water, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 26 μm was produced and used as a base material layer (2). Next, the polymer stock solution C is sprayed on one side of the base material layer (2) using an electrostatic spinning method (applied voltage: 12 kV, distance of 10 cm between the spinneret and the base material layer (2)), and the thickness is about 3 μm. After forming the ultrafine fiber layer, a super calender treatment was performed to obtain a lithium ion secondary battery separator (weight per unit: 12.7 g / m ² , thickness: 19 μm).

(Example 7)
For the base material layer (3), 20 parts of polypropylene short fibers having a fineness of 0.1 dtex and a fiber length of 3 mm, 40 parts of unstretched PET heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm, the above method 40 parts of the fibrillated lyocell fiber produced in the above were mixed together, disaggregated in pulper water, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 24 μm was produced and used as a base material layer (3). Next, the polymer stock solution A is sprayed on one side of the base material layer (3) using an electrostatic spinning method (applied voltage: 12 kV, distance of 15 cm between the spinneret and the base material layer (3)), and the thickness is about 4 μm. The ultrafine fiber layer of was formed. In this state, since the degree of crystallinity is low and soluble in water, a heat treatment (150 ° C., 3 min) is performed, followed by a supercalendering process to obtain a separator for a lithium ion secondary battery (weight per unit: 13.4 g / m ² , thickness 21 μm).

(Example 8)
For the base material layer (3), 20 parts of polypropylene short fibers having a fineness of 0.1 dtex and a fiber length of 3 mm, 40 parts of unstretched PET heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm, the above method 40 parts of the fibrillated lyocell fiber produced in the above were mixed together, disaggregated in pulper water, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 24 μm was produced and used as a base material layer (3). Next, the polymer stock solution B is sprayed on one surface of the base material layer (3) using an electrostatic spinning method (applied voltage: 15 kV, distance of 15 cm between the spinneret and the base material layer (3)), and the thickness is about 3 μm. After forming the ultrafine fiber layer, a super calender treatment was performed to obtain a lithium ion secondary battery separator (weight per unit area: 12.6 g / m ² , thickness: 20 μm).

Example 9
For the base material layer (3), 20 parts of polypropylene short fibers having a fineness of 0.1 dtex and a fiber length of 3 mm, 40 parts of unstretched PET heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm, the above method 40 parts of the fibrillated lyocell fiber produced in the above were mixed together, disaggregated in pulper water, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 24 μm was produced and used as a base material layer (3). Next, the polymer stock solution C is sprayed on one side of the base material layer (3) using an electrostatic spinning method (applied voltage: 12 kV, distance of 10 cm between the spinning port and the base material layer (3)), and the thickness is about 3 μm. After forming the ultrafine fiber layer, a super calender treatment was performed to obtain a lithium ion secondary battery separator (weight per unit area: 12.5 g / m ² , thickness: 20 μm).

(Example 10)
For the base material layer (4), 10 parts of PET short fibers oriented and crystallized with a fineness of 0.06 dtex and a fiber length of 3 mm, 10 parts of PET short fibers oriented and crystallized with a fineness of 0.1 dtex and a fiber length of 3 mm are used. 40 parts of unstretched PET-based heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm, and 40 parts of fibrillated lyocell fibers prepared by the above method are mixed together and disaggregated in water of a pulper. A uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 24 μm was produced and used as a base material layer (4). Next, the polymer stock solution A is sprayed on one side of the base material layer (4) using an electrostatic spinning method (applied voltage: 12 kV, distance of 15 cm between the spinning port and the base material layer (4)), and the thickness is about 4 μm. The ultrafine fiber layer of was formed. In this state, since the degree of crystallinity is low and soluble in water, a heat treatment (150 ° C., 3 min) is performed, followed by a supercalendering process to obtain a lithium ion secondary battery separator (weight: 13.1 g / m ² , thickness 21 μm).

(Comparative Example 1)
20 parts of oriented and crystallized polyethylene terephthalate (PET) short fibers having a fineness of 0.1 dtex and a fiber length of 3 mm, 40 parts of unstretched PET heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm, the above method 40 parts of the fibrillated lyocell fiber produced in the above were mixed together, disaggregated in pulper water, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This slurry for paper making is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop non-woven fabric strength, followed by super calender treatment A nonwoven fabric having a basis weight of 10.1 g / m ² and a thickness of 17 μm was manufactured, and a separator for a lithium ion secondary battery (a basis weight of 12.1 g / m ² , a thickness of 20 μm) was obtained.

(Comparative Example 2)
20 parts of acrylic short fibers having a fineness of 0.1 dtex and a fiber length of 3 mm, 40 parts of unstretched PET heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm, and 40 parts of fibrillated lyocell fibers prepared by the above method. Were mixed together, disaggregated in water of a pulper, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This slurry for paper making is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop non-woven fabric strength, followed by super calender treatment A nonwoven fabric having a basis weight of 10.3 g / m ² and a thickness of 16 μm was produced, and a separator for a lithium ion secondary battery (a basis weight of 11.8 g / m ² and a thickness of 21 μm) was obtained.

(Comparative Example 3)
20 parts of polypropylene short fibers having a fineness of 0.1 dtex and a fiber length of 3 mm, 40 parts of unstretched PET heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm, and 40 parts of fibrillated lyocell fibers prepared by the above method. Were mixed together, disaggregated in water of a pulper, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This slurry for paper making is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop non-woven fabric strength, followed by super calender treatment A nonwoven fabric having a basis weight of 10.1 g / m ² and a thickness of 17 μm was produced, and a separator for a lithium ion secondary battery (a basis weight of 12.0 g / m ² , a thickness of 20 μm) was obtained.

(Comparative Example 4)
The polymer stock solution A is sprayed onto the surface of the stainless steel drum collector using an electrostatic spinning method (applied voltage 12 kV, distance 15 cm between the spinneret and the base material layer (3)) to form a super fine fiber layer having a thickness of 22 μm. Formed. In this state, the degree of crystallinity is low and soluble in water. Next, after heat treatment (150 ° C., 3 min), supercalender treatment was performed, and a lithium ion secondary battery separator (weight per unit 12.2 g / m ² , thickness 18 μm).

(Comparative Example 5)
The polymer stock solution B is sprayed onto the surface of the stainless steel drum collector using an electrostatic spinning method (applied voltage: 15 kV, distance of 15 cm between the spinneret and the base material layer (3)) to form a superfine fiber layer having a thickness of 22 μm. After the formation, a super calender treatment was performed to obtain a lithium ion secondary battery separator (weight per unit area: 12.5 g / m ² , thickness: 17 μm).

(Comparative Example 6)
The polymer stock solution C is sprayed onto the surface of the stainless steel drum collector using an electrostatic spinning method (applied voltage: 12 kV, distance of 10 cm between the spinneret and the base material layer (3)) to form a superfine fiber layer having a thickness of 24 μm. After the formation, a super calender treatment was performed to obtain a lithium ion secondary battery separator (weight per unit area: 13.2 g / m ² , thickness: 19 μm).

(Comparative Example 7)
For base material layer (5), 60 parts of oriented and crystallized polyethylene terephthalate (PET) short fibers with a fineness of 0.1 dtex and fiber length of 3 mm, unstretched PET heat fusion with a fineness of 0.2 dtex and fiber length of 3 mm 40 parts of the characteristic short fibers were mixed together, disaggregated in water of a pulper, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 25 μm was produced and used as a base material layer (5). Next, the polymer stock solution A is sprayed on one surface of the base material layer (5) using an electrostatic spinning method (applied voltage: 12 kV, distance of 15 cm between the spinneret and the base material layer (5)), and the thickness is about 4 μm. The ultrafine fiber layer of was formed. In this state, since the crystallinity is low and soluble in water, a heat treatment (150 ° C., 3 min) is performed, followed by a super calender treatment to obtain a lithium ion secondary battery separator (weight per unit: 12.7 g / m ² , thickness 21 μm).

(Comparative Example 8)
For the base material layer (6), 60 parts of acrylic short fibers having a fineness of 0.1 dtex and a fiber length of 3 mm, and 40 parts of unstretched PET heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm are mixed together. Then, the pulper was disaggregated in water, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. This papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric having a thickness of m ² and a thickness of 26 μm was produced and used as a base material layer (6). Next, the polymer stock solution A is sprayed on one surface of the base material layer (6) using an electrostatic spinning method (applied voltage: 12 kV, distance of 15 cm between the spinneret and the base material layer (6)), and the thickness is about 4 μm. The ultrafine fiber layer of was formed. In this state, since the crystallinity is low and soluble in water, a heat treatment (150 ° C., 3 min) is performed, followed by a super calender treatment to obtain a lithium ion secondary battery separator (weight per unit: 13.2 g / m ² , thickness 21 μm).

(Comparative Example 9)
For the base material layer (7), 60 parts of polypropylene short fibers having a fineness of 0.08 dtex and a fiber length of 3 mm, and 40 parts of unstretched PET heat-fusible short fibers having a fineness of 0.2 dtex and a fiber length of 3 mm are mixed together. Then, the pulper was disaggregated in water, and a uniform papermaking slurry (1% concentration) was prepared under stirring by an agitator. The papermaking slurry is made up by a wet method using a circular paper machine, and unstretched PET-based heat-adhesive short fibers are adhered by a cylinder dryer at 120 ° C. to develop a nonwoven fabric strength. A non-woven fabric of m ² and a thickness of 26 μm was produced and used as a base material layer (7). Next, the polymer stock solution A is sprayed on one side of the base material layer (7) using an electrostatic spinning method (applied voltage: 12 kV, distance of 15 cm between the spinneret and the base material layer (7)), and the thickness is about 4 μm. The ultrafine fiber layer of was formed. In this state, since the degree of crystallinity is low and soluble in water, a heat treatment (150 ° C., 3 min) is performed, followed by a supercalendering process to obtain a lithium ion secondary battery separator (weight: 13.1 g / m ² , thickness 21 μm).

<Evaluation>
The lithium ion secondary battery separators obtained in the examples and comparative examples were evaluated as follows, and the evaluation results are shown in Table 1.

[Evaluation of pinholes]
Fifty test pieces of 200 mm × 200 mm were collected from the lithium ion secondary battery separator, the surface of each test piece was visually observed, and the presence or absence of pinholes was measured to evaluate in the following two stages.
○: No pinhole is observed in all specimens.
X: Pinholes are observed in two or more of all the test pieces.

[Evaluation of puncture strength]
A 50 mm × 200 mm test piece is taken from a lithium ion secondary battery separator and mounted on a 40 mmφ fixed frame mounted on a desktop material testing machine (trade name: STA-1150, manufactured by Orientec Co., Ltd.). Then, a metal needle having a diameter of 1.0 mm (made by Orientec Co., Ltd.) with a rounded end (curvature 1.6) was lowered at a constant speed of 50 mm / min perpendicular to the sample surface. The maximum load (g) at this time was measured and used as the puncture strength. The puncture strength was measured at five or more locations for one sample, and the smallest puncture strength among all the measured values was indicated as ◯ if it was 50 g or more, Δ if it was 20 g or more and less than 50 g, and × if it was less than 20 g.

[Evaluation of delamination]
By visually observing whether the fibers constituting the base material layer and the ultrafine fiber layer have sufficiently penetrated into each other's inner layers, and by rubbing the ultrafine fiber layer by hand, in the following two steps evaluated.
○: The fibers sufficiently penetrate into each other's inner layer, and no delamination occurs even when rubbed by hand.
X: Fibers do not enter the inner layers of each other, and delamination easily occurs when rubbing by hand.

[Evaluation of heat resistance]
The separator for lithium ion secondary batteries was placed in a thermostatic bath at 150 ° C., subjected to heat treatment for 40 minutes, and the shrinkage rate of each separator for lithium ion secondary batteries was measured to evaluate the heat resistance. The shrinkage rate was measured as follows. A 50 mm x 50 mm sheet sample is cut out, the CD side of the sample is fixed with a clip, sandwiched between heat resistant glass plates, stored for 40 minutes in a thermostatic bath at 150 ° C, taken out, and the length of the sheet sample is measured and tested. Compared with the previous length, the percentage of the length reduction rate was taken as the shrinkage rate. A value of less than 5% was evaluated as ○, a value of 5% or more and less than 8% was evaluated as Δ, and a value of 8% or more was evaluated as x. In addition, when the heat resistance of a 20 μm thick polyethylene microporous membrane, which is a conventionally known lithium ion secondary battery separator, was evaluated, the polyethylene microporous membrane melted and contracted, and the shrinkage ratio was 30% or more. Met.

  In order to evaluate the electrical characteristics of the separators for lithium ion secondary batteries obtained in the examples and comparative examples, the following electrodes and batteries were prepared and measured.

<Preparation of positive electrode>
Uniformly, 75 parts by mass of lithium cobaltate as a positive electrode active material, 15 parts by mass of acetylene black as a conductive additive, and 5 parts by mass of polyvinylidene fluoride (PVdF) as a binder are uniformly contained in N-methyl-2-pyrrolidone (NMP). To obtain a positive electrode paste. This paste was applied onto an aluminum foil having a thickness of 22 μm, dried and calendered to produce a positive electrode having a thickness of 97 μm.

<Production of negative electrode>
A negative electrode agent paste was prepared by uniformly mixing 95 parts by mass of graphite as a negative electrode active material and 5 parts by mass of PVdF as a binder using NMP as a solvent. This negative electrode paste was applied onto a copper foil having a thickness of 24 μm, dried and calendered to produce a negative electrode having a thickness of 92 μm.

<Production of battery>
The positive electrode and the negative electrode obtained as described above are overlapped with each other through the separators for lithium ion secondary batteries of Examples and Comparative Examples, and loaded into a laminate film outer packaging material, and 1 mol / L LiBF ₄ as an electrolyte. A lithium ion secondary battery was manufactured by injecting an ethylene carbonate / diethyl carbonate (volume ratio: 1/1) solution in which the solution was dissolved and vacuum-sealing.

[Evaluation of internal resistance]
The internal resistance of the manufactured lithium ion secondary battery was measured by the AC impedance method under the conditions of an amplitude of 10 mV and a frequency of 10 kHz. When the internal resistance value is less than 1.35Ω, it is indicated by “◯”, when it is 1.35Ω or more and less than 1.55Ω, Δ is indicated, and when it is 1.55Ω or more, × is indicated.

[Evaluation of discharge capacity maintenance rate]
The produced lithium ion secondary battery was subjected to constant current charging at 1C (up to 4.1V) and constant voltage charging at 4.1V, and repeated constant current discharging up to 3.0V at 1C. The ratio of the discharge capacity at the 100th time relative to the above was determined as a percentage (%), and this was used as the discharge capacity maintenance ratio. When the discharge capacity retention rate was 90% or more, it was indicated by “◯”, when it was 80% or more and less than 90%, “Δ”, and when it was less than 60%, it was indicated by “X”.

  The separator for lithium ion secondary batteries obtained in Examples 1 to 10 includes a base material layer containing synthetic resin short fibers and fibrillated lyocell fibers, and an ultrafine fiber layer obtained by an electrospinning method. The result was that the two layers were not easily peeled off, had no pinhole defects, had not only excellent heat resistance at 150 ° C., but also extremely excellent battery characteristics.

  On the other hand, since the separator for lithium ion secondary batteries obtained in Comparative Examples 1 to 3 is composed only of the base material layer, it has a pinhole defect and is not suitable as a separator for lithium ion secondary batteries. Met. Since the separators for lithium ion secondary batteries obtained in Comparative Examples 4 to 6 were composed of only the ultrafine fiber layer obtained by the electrostatic spinning method, the results were inferior to the puncture strength and the battery characteristics. Moreover, since the separator for lithium ion secondary batteries obtained in Comparative Examples 7 to 9 does not contain the fibrillated lyocell fiber in the base material layer, the entanglement between the fibers with the ultrafine fiber layer is weak and peeling. As a result, the battery characteristics were greatly inferior to those of the examples.

  As an application example of the present invention, it is suitably used for a separator for a lithium ion secondary battery such as a lithium ion secondary battery or a lithium ion polymer secondary battery, and can also be used as a separator for a lithium ion capacitor.

Claims (1)

  It has a base material layer containing synthetic resin short fibers and fibrillated lyocell fibers as essential components, and an ultrafine fiber layer obtained by an electrospinning method is directly deposited and laminated on the base material layer The separator for lithium ion secondary batteries characterized by the above-mentioned.